This invention relates to a safety device for hydraulic jacks, particularly to a separate overload detecting and warning circuit system disposed in the hydraulic jack for ensuring complete safety in operation.
As hydraulic jacks are widely used in lifting operations for ordinary mechanical work, the effort put toward safe construction of hydraulic jacks is tremendous and important to both life and property. Accordingly, the structure of the conventional high-grade hydraulic jacks at present usually takes the safety coefficient of the hydraulic cylinder wall into consideration by providing a safety valve in the high-pressure piping thereof so as to prevent the check valve of the high-pressure cylinder from being jammed (resulting from external overload thereof) in case of an abnormal high-pressure state occurring in the high-pressure cylinder. In this condition, the hydraulic fluid continuously pumped therefrom and compressed in the high-pressure piping will force the safety valve arranged with weak resistance thereto to pop open and let the hydraulic fluid return to the low-pressure cylinder so as to avoid the failure of the high-pressure cylinder as a result of unlimited rise of the pressure applied thereto. However, even with the safety limit marked on the conventional hydraulic jack for ensuring safe operation, the user usually takes a hydraulic jack with small capacity to perform the lifting operation requiring a greater capacity. The problem is that: (1) the user cannot exactly estimate the weight of the load; and (2) while the overload condition is developing on the hydraulic jack, no warning indication is given to the user. Moreover, as the aforesaid safety valve is usually provided at a place outside of the high-pressure cylinder, when the load on the hydraulic jack is suddenly increased (such as a worker casually climbing onto the load), a hazardous high pressure will be abruptly produced in the high pressure cylinder. In this case, not only the abnormal high pressure cannot be relieved, but also the worker operating the jack is not aware of this dangerous situation.
For overcoming the aforesaid defects of the conventional jacks, this inventor has designed an "Overload Detecting Device" and filed a patent application in Taiwan on Oct. 17, 1981, which application was granted as a Utility Model patent on July 22, 1982 with the Pat. No. 16450. The same application was also filed in the Patent and Trademark Office of the United States on Nov. 6, 1981 with the Ser. No. 319,382, which issued as U.S. Pat. No. 4,417,236. After reviewing the practical operation and making a further study of the above-mentioned invention, however, this inventor has again found that the technical disclosure of the previously designed "Overload Detecting Device" has a performance limit, i.e., it cannot provide the hydraulic jack with separate and multiple load-limit settings in overload sensing and warning operations.